Messages in this thread | | | Date | Fri, 20 Apr 2018 11:25:40 +0200 | From | Juri Lelli <> | Subject | Re: [PATCH] sched/rt: Rework for_each_process_thread() iterations in tg_has_rt_tasks() |
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Hi Kirill,
On 19/04/18 20:29, Kirill Tkhai wrote: > tg_rt_schedulable() iterates over all child task groups, > while tg_has_rt_tasks() iterates over all linked tasks. > In case of systems with big number of tasks, this may > take a lot of time. > > I observed hard LOCKUP on machine with 20000+ processes > after write to "cpu.rt_period_us" of cpu cgroup with > 39 children. The problem occurred because of tasklist_lock > is held for a long time and other processes can't do fork(). > > PID: 1036268 TASK: ffff88766c310000 CPU: 36 COMMAND: "criu" > #0 [ffff887f7f408e48] crash_nmi_callback at ffffffff81050601 > #1 [ffff887f7f408e58] nmi_handle at ffffffff816e0cc7 > #2 [ffff887f7f408eb0] do_nmi at ffffffff816e0fb0 > #3 [ffff887f7f408ef0] end_repeat_nmi at ffffffff816e00b9 > [exception RIP: tg_rt_schedulable+463] > RIP: ffffffff810bf49f RSP: ffff886537ad7d50 RFLAGS: 00000202 > RAX: 0000000000000000 RBX: 000000003b9aca00 RCX: ffff883e9cb4b1b0 > RDX: ffff887d0be43608 RSI: ffff886537ad7dd8 RDI: ffff8840a6ad0000 > RBP: ffff886537ad7d68 R8: ffff887d0be431b0 R9: 00000000000e7ef0 > R10: ffff88164fc39400 R11: 0000000000023380 R12: ffffffff81ef8d00 > R13: ffffffff810bea40 R14: 0000000000000000 R15: ffff8840a6ad0000 > ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018 > --- <NMI exception stack> --- > #4 [ffff886537ad7d50] tg_rt_schedulable at ffffffff810bf49f > #5 [ffff886537ad7d70] walk_tg_tree_from at ffffffff810c6c91 > #6 [ffff886537ad7dc0] tg_set_rt_bandwidth at ffffffff810c6dd0 > #7 [ffff886537ad7e28] cpu_rt_period_write_uint at ffffffff810c6eea > #8 [ffff886537ad7e38] cgroup_file_write at ffffffff8111cfd3 > #9 [ffff886537ad7ec8] vfs_write at ffffffff8121eced > #10 [ffff886537ad7f08] sys_write at ffffffff8121faff > #11 [ffff886537ad7f50] system_call_fastpath at ffffffff816e8a7d > > The patch reworks tg_has_rt_tasks() and makes it to check > for rt_rq::rt_nr_running instead of iteration over task list. > This makes the function to scale well, and its execution time > does not depend on number of processes in the system. > > Note, that since tasklist_lock doesn't protect a task against > sched_class changing, we don't introduce new races in comparison > to that we had before. Also, rt_rq::rt_nr_running contains queued > child cfs_rq in additional to queued task. Since tg_has_rt_tasks()
s/cfs_/rt_/ , right?
> is used in case of !runtime case: > > if (rt_bandwidth_enabled() && !runtime && tg_has_rt_tasks(tg)) > return -EBUSY; > > the behaviour won't change. The only change is that walk_tg_tree() > calling tg_rt_schedulable() will break its iteration on parent cfs_rq,
Ditto.
> i.e. earlier. > > Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com> > --- > kernel/sched/rt.c | 22 ++++++++++++---------- > 1 file changed, 12 insertions(+), 10 deletions(-) > > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c > index 7aef6b4e885a..601151bb9322 100644 > --- a/kernel/sched/rt.c > +++ b/kernel/sched/rt.c > @@ -2395,10 +2395,10 @@ const struct sched_class rt_sched_class = { > */ > static DEFINE_MUTEX(rt_constraints_mutex); > > -/* Must be called with tasklist_lock held */ > static inline int tg_has_rt_tasks(struct task_group *tg) > { > - struct task_struct *g, *p; > + struct rt_rq *rt_rq; > + int cpu, ret = 0; > > /* > * Autogroups do not have RT tasks; see autogroup_create(). > @@ -2406,12 +2406,18 @@ static inline int tg_has_rt_tasks(struct task_group *tg) > if (task_group_is_autogroup(tg)) > return 0; > > - for_each_process_thread(g, p) { > - if (rt_task(p) && task_group(p) == tg) > - return 1; > + preempt_disable(); > + > + for_each_online_cpu(cpu) { > + rt_rq = tg->rt_rq[cpu]; > + if (READ_ONCE(rt_rq->rt_nr_running)) {
Isn't this however checking against the current (dynamic) number of runnable tasks/groups instead of the "static" group membership (which shouldn't be affected by a task running state)?
Best,
- Juri
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